Conventional hydraulic elevators include a hydraulically driven ram to raise an elevator car. Lowering of the car is typically accomplished by permitting fluid to exit the cylinder of the hydraulic ram and using the weight of the car to force the fluid out of the cylinder. A piston of the hydraulic ram may be directly engaged with the car or may be engaged with the car via a rope fixed to the hoistway and engaged with a sheave on a yoke on the piston. The latter arrangement provides the benefit of not requiring space under the hoistway for the hydraulic cylinder, although at the price of requiring additional space adjacent to the travel path of the car.
One advantage of hydraulic elevators as compared to traction elevators is the lower cost of the installation. A disadvantage, however, is the higher power requirements for the hydraulic pump as compared to similar sized traction elevators. This is in part the result of the hydraulic ram having to carry the weight of the car and the passenger load.
One method to reduce the power requirements of hydraulic elevators is to use a counterweight, as is done with traction elevators. In U.S. Pat. No. 5,238,087, issued to Garrido et al and entitled "Advanced Energy Saving Hydraulic Elevator", a double-acting hydraulic cylinder is used with a counterweighted hydraulic elevator. The double-acting hydraulic cylinder permits the car to driven in both the upward and downward direction, thus allowing the counterweight to be heavier than the empty car. The double-acting cylinder is more expensive than a single-acting hydraulic cylinder and requires more complex control of the hydraulic elevator. In another example disclosed in U.S. Pat. No. 5,014,823, issued to Pelto-Huikko and entitled "Apparatus for Improving the Performance of a Motor-Controlled Hydraulic Elevator", a single-acting hydraulic cylinder is used with a counterweight directly engaged with the car. This proposed solution requires additional hoistway space to accommodate the counterweight, thus minimizing the benefits.
The above art notwithstanding, engineers under the direction of Applicant's Assignee are working to develop inexpensive hydraulic elevators that minimize power requirements and hoistway space.